Heat-shock proteins are induced by high temperatures and many toxic agents. They play important roles in maintaining cell viability under stressful environmental conditions. Many of the proteins also play vital roles at normal temperatures, helping other proteins to fold, associate into complexes, and arrive at their proper destinations. This proposal focuses on the highly conserved hsp9O family of proteins, which are found in virtually all plant, animal, and microbial cells. A great deal of biochemical analysis in vertebrate systems has established that the proteins in the hsp9O family associate with a diverse array of other proteins, including steroid hormone receptors, a variety of kinases (most notably the ongogenic tyrosine kinases), actin, and tubulin. Genetically, the functions of the proteins are best characterized in the yeast, Saccharomyces cerevisiae. The genome of this organism encodes two closely related proteins in this family. Together, the proteins constitute an essential gene pair. Cells can live without either protein, but die in the absence of both. The goal of this proposal is to employ the powerful methods of genetic analysis in yeast to analyze the molecular functions of hsp9O proteins. A major focus is to.import previously characterized vertebrate proteins into yeast cells and determine how their functions are affected by associations with hsp9O. The proteins of most immediate interest are the steroid hormone receptors and the oncogenic tyrosine kinases. Other aims include identifying natural yeast targets for hsp9O and determining the functional significance of accessory proteins that are characteristically found in hsp9O protein complexes.